Released staapl-0.5.11 (PLaneT 1.11)

As usual, there is a main tarball release which includes example code,
and a PLaneT Racket module that contains just documentation and
modules.
Main changes:
- Microchip PicKit 2 support.
Hook up a PK2 to a factory-clean PIC chip and get started. See
app/Makefile in the .tar.gz distribution.
- Implemented Racket unit support.
It seems that Racket's unit system (pluggable components) is a better
abstraction for Forth code modules, as opposed to Racket's module
system. This release contains a major refactoring of the Coma
compiler and Forth frontend to support standard wordset signatures.